1. Field of the Invention
The present invention relates to an optical filter using guided mode resonance.
2. Description of the Related Art
In recent years, separately from the conventional color filter using pigment, a color filter using guided mode resonance (GMR) or Guided Resonance (GR) Mode has been proposed. The color filter using guided mode resonance takes advantage of a resonance condition between diffracted light and a waveguide mode occurring in a waveguide. Optical filters based on GMR are considered to provide high light use efficiency and a sharp resonance spectrum.
Kanamori et al., in “Fabrication of Transmission Color Filters Using Silicon Subwavelength Gratings on Quartz Substrates”, IEEE Photonics Technology Letters, Vol. 18, No. 20, Oct. 15, 2006 (hereinafter referred to as Literature 1) discloses a color filter that includes a one-dimensional diffraction grating formed of silicon and uses the guided mode resonance occurring in the silicon periodic structure.
The filter disclosed in Literature 1 transmits RGB (Red, Green, and Blue) portions of the incident light having a particular polarization component. TE polarized light is made to be incident on a filter for red and a filter for green, and TM polarized light is made to be incident on a filter for blue.
Japanese Patent Laid-Open No. 2007-41555 (hereinafter referred to as Literature 2) discloses a filter that includes a two-dimensional diffraction grating formed of silicon and transmits R and G light. Literature 2 states that making the diffraction grating perfectly two-dimensional can reduce polarization dependency.
The color filter disclosed in Literature 1 uses a one-dimensional diffraction grating, therefore the angle between the grating structure of the diffraction grating and polarized light incident thereon may have various values depending on the angle of incident polarized light, and therefore the transmission property thereof has strong polarization dependency. Such strong polarization dependency causes inconvenience, for example, when the filter is used in an image sensor of a digital camera or the like.
Although Literature 2 discloses that use of a two-dimensional diffraction grating reduces the polarization dependency of color filters for red and green, a color filter for blue, which constitutes the three primary colors, is not achieved.
In view of the use in an image sensor for picking up a color image, it is desirable to achieve a color filter that transmits the three primary colors including blue with a high transmittance.